I. Field of the Invention
This invention relates to weighing devices, and more particularly to low range automatic balance devices having an optical readout system.
II. Description of the Prior Art
Many low range type of scales and balances have been devised and described in the prior art. Many of these devices, however, exhibit numerous problems. A basic disadvantage of a scale as compared to a balance is that scales are sensitive to regional gravitational variations and temperature variations. Several of the low range type of scales and balances described in the prior art do not have the degree of accuracy in weighing that is required in various situations, such as the weighing requirements that exist within the postal field. Furthermore, many of the known weighing devices are not automatic and do not provide for a direct visual read-out of the weight of the object being weighed. In addition, many of these devices are relatively complex in design, difficult to manufacture, require the use of high cost materials and are expensive.
One example of a prior art type of low range scale is described in U.S. Pat. No. 2,716,546. The scale illustrated by this patent, when in a rest position, has a plurality of counterweights (balls) supported on carrier by a spring. When a weight (e.g., a letter) is placed on the weighing pan, the spring extends, thereby successively depositing balls on a plurality of supports which are of different vertical heights so that during the descent of the weighing pan, balls are successively deposited from a shelf onto the supports in a one after the other fashion. A linkage connected to the shelf, and responsive to its vertical height, operates a pointer for indicating the weight on the pan. The scale includes a bimetal strip for compensating for variations in the force of the spring resulting from changes in temperature. The sensitivity of the spring and the required adjusting of each of the counterweights to a particular height are two of the problems related to this type of scale which can alter the accuracy of any weight measurements made thereon. Other examples of scales disclosed by the prior art and relating to the present invention are illustrated in U.S. Pat. Nos. 213,898; 276,701; 1,661,556; 1,944,071; 3,108,647; 4,106,580; and 4,235,296.
In commonly assigned and copending application Ser. No. 361,310, filed Mar. 24, 1982 and entitled "A Low Range Automatic Weighing Device" by D. W. Hubbard there is disclosed a low range automatic balance which overcomes many of the disadvantages of the type of low range weighing devices disclosed in the prior art. Basically, this device comprises a frame, a pan element for supporting a load to be weighed, a channel member connected to the pan for retaining a plurality of weight elements in a vertically movable manner, a motor supported within the frame and a shaft member operatively connected to and extending from the motor and supporting the pan element. A plurality of support members are provided that have top end portions adapted to receive and lift one of the weight elements. A camming arm is secured to the shaft member and extends in a direction toward the support members. A switch is provided for energizing the motor when the pan is depressed by a load. The motor turns the shaft member and guides the camming arm into contact with successive support members in a step-by-step manner to place a number of the support members in an extended position and lift a corresponding number of the weights substantially equal to the weight of the load. Because of the order in which the weight elements are positioned on the pan it is possible that this can result in a slight uneven weight distribution on the pan. This can tend to increase the friction between the motor shaft and the pan.
Alternate type of weighing devices which have resolved the above-described type of problems are described in my commonly assigned and copending application Ser. Nos. 395,775, filed July 6, 1982 and 395,777, filed July 6, 1982, both entitled "Low Range Automatic Weighing Device".